fix(priority-sync): handle missing rate data and backfill target group on re-import

P1 - Missing rate data now skips account instead of falling back to 1.0:
  In sync_account_priorities_for_upstream(), the rated list now filters
  out accounts whose upstream snapshot has no rate entry for their group_id.
  If after filtering a competitive bucket has fewer than 2 accounts with
  valid rate data, the entire bucket is silently skipped (no update_account
  call, no webhook) rather than treating missing rates as 1.0 and
  potentially triggering spurious notifications.

P2 - Re-importing an existing account now backfills imported_target_group_id:
  In the exists-is-True idempotency branch of import_upstream_keys_as_accounts(),
  if the current request supplies a target_group_id for the account's source group
  and it differs from what is stored, the field is written back and committed.
  This lets operators fix old data by simply re-running the import dialog.

Tests added:
  - test_missing_rate_skips_entire_competitive_group: all accounts in
    competitive group lack snapshot → bucket skipped, no update called
  - test_partial_missing_rate_sufficient_accounts_still_updates: 3 accounts
    in same bucket, 1 missing rate → the 2 with rates still compete normally

All 27 tests pass.

backend/app/services/website_sync.py

@@ -372,11 +372,19 @@ def sync_account_priorities_for_upstream(db: Session, upstream_id: int) -> list[
         # priority_assignment: account_id → new_priority
         priority_assignment: dict[str, int] = {}
         for comp_key, comp_rows in competitive_buckets.items():
-            # 取每行的倍率（查不到则 fallback 1.0）
+            # 只保留快照中能查到倍率的账号；无数据的账号不参与排序
             rated = [
-                (row, raw_rate_map.get(f"{row.upstream_id}:{row.group_id}", 1.0))
+                (row, raw_rate_map[f"{row.upstream_id}:{row.group_id}"])
                 for row in comp_rows
+                if f"{row.upstream_id}:{row.group_id}" in raw_rate_map
             ]
+            # 过滤后有效账号不足 2 个 → 此分组无竞争意义，整组跳过
+            if len(rated) < 2:
+                logger.info(
+                    "skip competitive bucket %s for website %s: only %d account(s) have rate data",
+                    comp_key, wid, len(rated),
+                )
+                continue
             # 组内按倍率升序排序（倍率低 → priority 小 → 优先）
             unique_rates = sorted(set(r for _, r in rated))
             rate_to_prio = {rate: idx + 1 for idx, rate in enumerate(unique_rates)}